Nerve-evoked Acetylcholine Release Research Articles

Facilitation by P 2 receptor activation of acetylcholine release from rat motor nerve terminals: interaction with presynaptic nicotinic receptors

ATP is released from motor nerve endings together with acetylcholine. Released adenine nucleotides can be extracellularly metabolized into adenosine, which is a presynaptic neuromodulator at neuromuscular junctions, but it is not known if P 2 receptor activation also modulates acetylcholine release from mature motor nerve endings. We now tested the effect of a stable ATP analogue, β,γ-imido ATP on the nerve-evoked release of acetylcholine from adult rat hemidiaphragm preparations. β,γ-Imido ATP (10–100 μM) facilitated in a concentration-dependent manner evoked acetylcholine release, and 30 μM β,γ-imido ATP caused a 125% facilitation of evoked acetylcholine release. This facilitatory effect of β,γ-imido ATP (30 μM) was abolished by the P 2 receptor antagonists, suramin (100 μM) and pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS, 10 μM), but not by the A 1 or A 2A adenosine receptor antagonists, 1,3-dipropyl-8-cyclopentylxanthine (50 nM) and ZM 241385 (50 nM), respectively. The facilitation of acetylcholine release by β,γ-imido ATP (30 μM) was also prevented by the nicotinic acetylcholine receptor antagonist, d-tubocurarine (1 μM) and the facilitatory effect (40%) of the nicotinic acetylcholine receptor agonist, 1,1-dimethyl-4-phenylpiperazinium (1 μM) was abolished by PPADS (10 μM). These results demonstrate a presynaptic facilitatory effect of P 2 receptor activation at the rat phrenic nerve endings, which is tightly coupled with the presynaptic nicotinic autofacilitatory system.

Counter Effects of Higenamine and Coryneine, Components of Aconite Root, on Acetylcholine Release from Motor Nerve Terminal in Mice

The counter effects of higenamine and coryneine, components of aconite root, on acetylcholine (ACh) release from motor nerve terminals in the mouse phrenic nerve–diaphragm muscle preparation were studied by a radioisotope method. Both nerve-evoked release and spontaneous release of [3H]-ACh from the preparation preloaded with [3H]-choline were measured. The change in the tetanic tension of muscle was simultaneously recorded in the same preparation. Higenamine (10 μM) augmented both the nerve-evoked and spontaneous ACh releases, and the muscle tension. The effects were inhibited by pretreatment with propranolol (10 μM), a β-adrenoceptor antagonist. Coryneine reduced the nerve-evoked release of ACh, accelerated the decay of tetanic tension (tetanic fade) at 30 μM, and it depressed the peak amplitude of tetanic tension at a higher concentration of 100 μM. These results suggest that of the two components contained in aconite root, higenamine increases ACh release via activation of β-adrenoceptor, and conversely coryneine depresses ACh release by preferentially acting at motor nerve terminal.

Effect of AH5183 (vesamicol) on cholinergic transmission in intact airway smooth muscle

The effect of the vesicular acetylcholine (ACh) transport blocker trans-2-(4-phenyl-piperidino)-cyclohexanol (AH5183) was studied in bronchial smooth muscle during activation of the vagus nerve. AH5183 inhibited in a dose-dependent manner the Ca 2+-sensitive electrically induced smooth muscle contractions in vitro with a half-inhibitory concentration (IC 50) of 1.6 ± 0.4 μM. The inhibition was complete within 68 ± 1 min ( n = 8) at approximately 20 μM AH5183 and was partly reversible after washing of the preparations. AH5183 (20 μM) reduced the level of endogenous ACh by 47.4 ± 7.6% ( n = 4) during this time period. The effect of AH5183 is most likely prejunctional, since the contractions induced post-junctionally by carbachol were not altered by AH5183. The irreversible anticholinesterase, soman, increased the tonus of airway smooth muscle as a result of accumulation of spontaneously released ACh from prejunctional leakage. AH5183 had no effect on this increase of muscle contraction. The present results show that the nerve-evoked release of ACh comes from an AH5183-sensitive pool, probably a vesicular pool, whereas leakage of ACh presumably comes from the cytoplasmic pool in airway smooth muscle.

A substance P-like peptide in bullfrog autonomic nerve terminals: Anatomy biochemistry and physiology

Substance P immunoreactivity was localized to dense-cored vesicles in parasympathetic preganglionic nerve terminals in the bullfrog cardiac ganglion. Analysis with radioimmunoassay and high pressure liquid chromatography indicated that the immunoreactive substance was more similar to substance P than to substance K but was not identical to either. The bullfrog substance P was also shown to be different than kassinin, eledoisin, ranatensin, bombesin, neuromedin K and physalaemin. Although the peptide was present in most of the terminals in the ganglion and appeared to be releasable in a calcium-dependent manner, intracellular recordings from ganglion neurons during stimulation revealed no electrophysiological events that might be mediated by an endogenous peptide. In addition, the direct application of substance P to ganglion neurons generally produced no changes in membrane potential, membrane conductance, somal calcium spikes or nerve-evoked release of acetylcholine. High concentrations of substance P did cause an apparent increase in the rate of desensitization of the nicotinic receptors but there was little indication that this phenomenon would occur under physiological conditions. It is suggested that bullfrog substance P is released as a neurotransmitter but is involved in electrically silent events in the postsynaptic neurons or is acting on non-neuronal cells near the terminals. The possible implications of these results for peptidergic systems in general are discussed.